The present invention generally relates to a common driver circuit for driving a large capacitance load such as a common electrode, or the like, of a liquid crystal panel.
Generally, as shown in FIG. 6, a common driver circuit built into a conventional liquid crystal display apparatus has a bias supplied through a variable resistor 2 to a buffer amplifier 1 to which a common amplitude signal (i.e., a frame pulse FP) is inputted, the amplitude of the common amplitude signal being adjusted by the adjustment of a bias. The output of the buffer amplifier 1 is such that the direct current component therein is removed by a capacitor C1 for such purpose and a common bias at the output is adapted to be supplied by a variable resistor 3. The above described common amplitude signal (FP) and the common bias via the variable resistor 3 are supplied to a common electrode of a liquid crystal panel.
In a conventional embodiment, in order to obtain an output amplitude signal to which a common bias is added, the direct current component had to be removed by the capacitor C1 at the output of the buffer amplifier 1. Therefore, in order to provide the common bias for each field of the liquid crystal display panel, a capacitor of large capacity was required as a capacitor C1. Also, as the common bias depends upon the power voltages Vcc, -Vee, to be applied to the variable resistor 3, the common bias can change and produce adverse effects upon the liquid crystal panel when these power voltages change.
As shown in FIG. 6, conventionally the direct current component in the common output was removed by the capacitor C1 after amplifying the frame pulse, and the common bias was added by a variable resistance. The amplitude of the common output was sometimes changed as much as 8 volts at its maximum. Also, the voltage -Vee connected with the common bias adjustment resistance 3 sometimes cannot be adjusted in relation to the source power voltage of the panel, depending upon the common bias which is to be applied. Therefore, the voltage -Vee was required to be a relatively low voltage (e.g., -21 V). Since the voltage -Vee varies independently of the variation in the source power voltage Vcc of the panel, a direct current component may be added to the panel when the source power voltage is not stable. When the voltage for providing the common bias is taken from the same power line as the source power of voltage Vcc of the panel, the bias to be set by resistance 3 varies in the same direction as the variation in the source power voltage, with a disadvantage that it is difficult to apply the direct current component to the panel. Also, in the specification for the panel, both variations in the central voltage of the video signal to be applied to the source driver and variations in the central voltage at the common bias are required to be controlled in accordance with the conditions of the signal content, the power voltage variations, and so on. In the video signal, feedback is already applied and such signal is stable, while in the common bias, the specification requirement was not necessarily observed because of the above described causes.